Organic Light Emitting Diodes (OLEDs) have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and may employ a plurality of pixels. The pixels are typically arranged in two-dimensional arrays with a row and a column address for each pixel and having a data value associated with the pixel value. However, such displays suffer from a variety of defects that limit the quality of the displays. In particular, OLED displays suffer from non-uniformities in the pixels. These non-uniformities can be attributed to both the light emitting materials in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the light emitting elements.
Referring to FIG. 2, in a current manufacturing and grading process, the OLED devices are first manufactured. This manufacturing step 100 involves, e.g., the preparation of a substrate, typically glass, the formation of electrodes and other electronic components on the substrate, the deposition of organic material layers, the addition of a second electrode, the encapsulation of the device, followed by singulation, packaging, and the attachment of electrical connectors. After the OLED device is manufactured, its performance is measured 110 to ensure that the light-emitting elements of the OLED device are working properly. Some faults may be present, for example stuck-on or stuck-off pixels, dark or bright pixels, and other non-uniform pixels. The OLED device may or may not meet the standards of the application for which it is intended so it is graded 120. If the OLED device does not meet the specification standards of the application, a repair 130 may be attempted. If the repair is not possible, the display is discarded 140. If it can be repaired, the repair is performed and the device tested 110 again.
If the OLED device does meet the application standards, it is a good device, and is subsequently burned-in 150 by illuminating the OLED device over a period of time with a burn-in pattern, for example a flat-field image. This burn-in process is necessary to ensure a stable operation of the device when it is first used in an application. Following burn-in, the device performance is again measured 160 and re-graded 170. If the device does not meet the specification at this point, it is discarded 180. If it does meet the specification, it may be shipped to a customer 190.
This process is effective but suffers from a high rejection rate. Some faults in light emitters may be compensated using a variety of means taught in the art. For example, copending, commonly assigned U.S. Ser. Nos. 10/858,260, 10/869,009 and 10/894,729 describe various means to detect and correct for some faults found in OLED devices. Other methods, for example, U.S. Pat. No. 6,414,661 B1 entitled “Method and apparatus for calibrating display devices and automatically compensating for loss in their efficiency over time” by Shen et al issued 20020702 describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes in an OLED display device by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. The compensation system is best used after the display device has been calibrated to provide uniform light output. This patent provides a means for correcting the non-uniformities through the use of a look-up table.
U.S. Pat. No. 6,473,065 B1 entitled “Methods of improving display uniformity of organic light emitting displays by calibrating individual pixel” by Fan issued 20021029 describes methods of improving the display uniformity of an OLED. In order to improve the display uniformity of an OLED, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction.
All of these correction schemes require uniformity and/or performance calibration information to be effective. However, the art does not teach manufacturing processes that provide a means to obtain the uniformity and/or performance calibration information in order to optimize the manufacturing process and thereby reducing the cost and improving the yield of the manufactured product.
There is a need, therefore, for an improved method of providing uniformity and reducing manufacturing costs in an OLED display manufacturing process.